In general, petroleum products largely change viscosities depending on temperatures, or they have temperature dependency in viscosity. In lubricating oils, for example, used in automobiles, temperature dependency in viscosity is preferably small. To lubricating oils are thus added a certain polymer soluble in a lubricant base oil as a viscosity modifier in order to reduce the temperature dependency in viscosity. Ethylene/α-olefin copolymers have recently been widely used as the viscosity modifier, and variously modified to improve a property balance of the lubricating oil (see, Patent Document 1).
Viscosity modifiers as described above are generally used for keeping an appropriate viscosity at high temperatures. Recently, with advancement in fuel efficiency as part of an effort to reduce environmental burdens, there has been a need for a viscosity-improving polymer capable of suppressing a viscosity increase particularly at lower temperature, or having excellent low-temperature characteristics. The polymers described in Patent Document 1 are effectively used, because keeping a polymer concentration as low as possible in general applications of lubricating oil is effective for achieving excellent low-temperature characteristics and advantageous in terms of economical efficiency.
However, in an application of lubricating oil for power transmission system, which is the field of the present invention, the lubricating oil is required to have more advanced low-temperature characteristics and shear stability, and to have quality based on the consideration for a balance of both performances. In this regard, the present inventors have studied and found that the viscosity modifiers described in Patent Document 1 still have a room for improvement.
In such a situation, as the result of extensive investigation, the present inventors have found that by using an ethylene/α-olefin copolymer having an ethylene content, a molecular weight, a molecular weight distribution, and a melting point each in specific ranges as a viscosity modifier of a lubricating oil for power transmission system, the problem described above is solved, and accomplished the present invention.    Patent Document 1: WO 00/34420 Pamphlet